1. Technical Field
The present invention relates to an oilfield tubing string and to a method for constructing the oilfield tubing string, and more particularly, the present invention relates to an oilfield tubing string having two passageways extending in fluid isolation therethrough and to a method for constructing the oilfield tubing string.
2. Description of Related Art
In many coiled tubing completions of hydrocarbon production wells, a safety valve is provided across the coiled production tube a predetermined distance beneath the surface to block the flow of produced hydrocarbon fluids through the production tube in the event of a tubing leak or other well failure downstream of the safety valve. Selective closure of the valve enables the operator to effectively prevent the escape of produced hydrocarbons into the surrounding environment should such a failure occur. Accordingly, the safety valve provides a means for reducing the environmental risk of hydrocarbon pollution when a hydrocarbon production well is placed in an environmentally sensitive locale, such as a wilderness area or an undersea aquatic habitat.
For offshore applications, placement of the subsurface safety valve is typically beneath, but relatively near, the sea floor. For onshore applications, placement of the safety valve is typically relatively shallow, i.e., within about twenty to forty meters of the earthen surface. Conventional subsurface safety valves are selectively actuated by means of a pressure signal transmitted to the valve from the surface. Accordingly, a service line, more specifically termed a control line, is provided to transmit the actuating pressure signal to the valve. The control line provides pressure communication between a pressure signal transmitter controlled by an operator at the surface and a valve actuator at the subsurface safety valve. The pressurizable control line extends from the surface to the safety valve alongside the production tube and has a cylindrical configuration with a convex external surface retaining a hydraulic or pneumatic fluid in the line.
The conventional safety valve is enclosed within a housing having an external nipple to receive the control line and having central inlet and outlet fittings to receive the ends of the production tube above and below the safety valve, respectively. The production tube is generally the primary conduit for producing hydrocarbons from the subterranean hydrocarbon-bearing formation to the surface. Accordingly, the production tube is considerably larger than the control line with respect to both the inside and outside diameters thereof. When running the control line alongside the production tube, it is common practice to bind the control line to the outside of the production tube with metal straps encircling the production tube and control line at periodically spaced intervals. The bound configuration simulates a unitary integrated tubing string that facilitates placement of the production tube and control line in the wellbore during completion of the well. The bound configuration, however, subsequently impairs the ability of the operator to perform successful workovers by limiting the available means for maintaining pressure control in the well.
Pressure control is conventionally maintained in a well during workovers by killing the well or alternatively, by preserving the live well, but pressure sealing the well with the production tube in place or stripped from the wellbore. Killing the well is the least desirable means for maintaining pressure control because it requires the placement of weighted kill fluids in the well that can damage the producing interval and diminish subsequent hydrocarbon production therefrom. Unfortunately, neither live means identified above for maintaining pressure control in the well is available when the control line is bound to the outside of the production tube. The bound configuration has an irregular outside surface with concavities formed therein resulting from the intersection of the cylindrical production tube and cylindrical control line. From a practical standpoint, the concavities prevent pressure sealing the well when the production tube and control line are retained in place within the well. The irregular surface of the bound configuration likewise prevents stripping the production tube and control line from the well to enable pressure sealing of the well thereafter. Thus, the presence of a coiled production tube and associated control line in a bound configuration undesirably dictates killing the well whenever a workover of the well is to be performed.
An alternate configuration for the coiled production tube and associated control line positions the control line internal to the production tube with the outside surface of the control line affixed to the inside surface of the production tube. Although the resulting structure has a convex outside surface free of concavities that enables pressure sealing of the well while the production tube and control line remain in place, the configuration is nevertheless unsatisfactory for well workovers. The presence of the cylindrical control line in the interior of the production tube impedes the passage of standard-size workover equipment, such as tools, plugs, perf guns, and the like, through the tube during workovers because the radially protruding control line inordinately diminishes the inside diameter of the tube. The cylindrical control line is also susceptible to damage or closure when workover equipment passes alongside it because the convex outside surface of the control line typically has a relatively thin wall thickness that is easily deformed when impinged against by workover equipment.
Accordingly, it is an object of the present invention to provide an integrated tubing string having a pair of pressure and/or fluid isolated passageways extending from a wellhead into a wellbore penetrating a subterranean hydrocarbon-bearing formation. It is a further object of the present invention to provide such a tubing string, wherein the first passageway is a tube for recovering hydrocarbon fluids from the formation or injecting hydrocarbon recovery fluids into the formation and the second passageway is a service line for communicating between the surface and the wellbore independent of the first passageway.
More particularly, it is an object of the present invention to provide an integrated tubing string containing a fluid passageway for transmitting fluids through a wellbore and containing a pressure passageway in pressure isolation from the fluid passageway for transmitting pressure signals through a wellbore. Still more particularly, it is an object of the present invention to provide a fluid production tube and an associated pressurizable control line that permits pressure maintenance of a live hydrocarbon production well while the production tube and control line remain in place during well workovers. It is another object of the present invention to provide a fluid production tube and an associated pressurizable control line that permits passage of standard-size workover equipment through the tube during well workovers. It is yet another object of the present invention to provide a pressurizable control line and a fluid production tube having a smooth concave inside surface contained within an integrated tubing string having a smooth convex outside surface. It is a further object of the present invention to provide a fluid production tube and a pressurizable control line contained within an integrated tubing string wherein the fluid production tube is sized in correspondence with conventional fluid production tubing.
It is also an object of the present invention to provide a process for constructing an integrated tubing string having a pair of pressure and/or fluid isolated passageways extending from a wellhead into a wellbore penetrating a subterranean hydrocarbon-bearing formation. It is more particularly an object of the present invention to provide a process for constructing an integrated tubing string containing a fluid passageway for transmitting fluids through a wellbore and containing a pressure passageway in fluid isolation from the fluid passageway for transmitting pressure signals through a wellbore. It is still more particularly an object of the present invention to provide a process for constructing a tubing string integrating a fluid production tube and a pressurizable control line therein, wherein the tubing string has a smooth outside surface and a smooth inside surface. It is yet a further object of the present invention to provide a process for constructing a tubing string integrating a fluid production tube and a pressurizable control line therein, wherein the fluid production tube is sized in correspondence with conventional production tubing. These objects and others are achieved in accordance with the invention described hereafter.